1. Field of the Invention
The present invention relates to a sheet conveyance device that has a function of conveying a sheet and a function of detecting a length of a sheet in the conveyance direction.
2. Description of the Related Art
In recent years, a copying machine used in an office is provided with an original reading device that conveys sheet-like originals, which are stacked on an original tray, one by one, and reads them automatically.
When originals are copied using the above-mentioned copying machine continuously, for example, it is necessary to select recording sheets of which sizes correspond to the original sizes. When a user designates a recording sheet, the designated recording sheet is used. On the other hand, when a recording sheet is automatically selected according to the original size, it is necessary to detect the size of the original at the time of reading the original. Accordingly, Japanese Laid-Open Patent Publication (Kokai) No. 2002-060089 (JP 2002-060089A) and Japanese Laid-Open Patent Publication (Kokai) No. 2005-150947 (JP 2005-150947A) disclose techniques for detecting an original length in a conveyance direction during conveyance of the original, for example.
JP 2002-060089A discloses an original reading device that detects the front end and the rear end of an original conveyed using a sensor arranged on an original-conveyance path, counts the number of driving pulses for a stepping motor for conveying an original between the detection timings of the front end and the rear end, and computes the length of the original in the conveyance direction based on the counted number of driving pulses.
The original reading device disclosed in JP 2005-150947A fixes a standard size by detecting a size of an original in a horizontal direction, which is perpendicular to the conveyance direction, during the conveyance of the original, and determines a setting number of driving pulses equivalent to the length of the standard size sheet in the conveyance direction. Then, the original reading device compares the setting number of driving pulses with the number of driving pulses applied to the stepping motor between the detection timings of the front and rear ends. When the difference of them is not larger than an allowed value, the device determines that the counted number of driving pulses becomes a new setting number of driving pulses. When the difference is larger than the allowed value, the device executes an error handling.
However, the device disclosed in JP 2002-060089A cannot detect the length of the original in the conveyance direction correctly when the rotating amount of an original feeding motor becomes larger than that of an ideal original conveyance due to slip of a conveyance roller to the original. In addition to the slip of the conveying roller, errors in accuracies of the conveying roller and the parts of an original-conveyance path, and meandering of an original within the original-conveyance path vary a conveyance state of an original, which obstructs a correct computation of the length of an original in the conveyance direction.
Moreover, since the device disclosed in JP 2005-150947A determines the setting number of driving pulses by finding the standard size of an original based on the width of the original, the correct setting numbers cannot be determined for originals of non-standard sizes that are increasing in recent years, such as checks. Moreover, when a user uses various sizes of originals, the setting numbers that should be stored for the respective original sizes also increase.